Abstract: In an automatic lathe, a touch trigger probe (4) is calibrated (datumed) against an item (6) having a known reference dimension, instead of against the lathe chuck (2) or against a surface machined in a workpiece. The calibrated probe is then used to touch two diametrically opposed points of a feature (11) centered with respect to the axis of rotation of the chuck (2). From the mean of the co-ordinates of the two points touched, there is calculated an offset related to the difference between the assumed axis of rotation and the actual axis of rotation. This calibrates the machine to compensate for any drift between the actual axis of rotation and the machine's measurement origin.